The present invention relates to a hardenable multi-part polymer composition comprising a radiopacifying filler, in particular but not exclusively, an acrylic composition which has at least two parts which react with each other upon being mixed together to form a cement such as a bone cement which hardens to a solid.
Hardenable compositions formed by mixing together acrylic polymers and monomers are useful in a wide range of applications. Particular utility is found in dental, medical, adhesive and construction applications, where such materials have been used for over 40 years.
Dental applications include denture bases, denture base plates, denture liners, denture repairs, custom trays, veneering for crowns and bridgework, artificial teeth, veneers and repair for natural teeth and tooth restorative fillings. Medical applications include bone cements for bone cavities such as prosthetic cements, cranial cements and vertebral cements in vertebroplasty and kyphoplasty. Further applications include the production of shaped articles that harden extra-corporeally and which can then be introduced into the body.
One type of hardenable acrylic compositions in which the invention is advantageous is generally composed of a solid component and liquid component. The solid component comprises a powder formed from polymer particles and, if appropriate, further additives, such as polymerisation initiators and catalysts, fillers and dyestuffs. The liquid component comprises a liquid monomer or monomers and further additives, such as accelerators and stabilisers. When ready for use, the solid and liquid components are mixed together to form a liquid or semi-solid paste, which, under the action of the polymerisation initiators and accelerators, increases in viscosity and hardens into a solid.
The solid component typically used consists of small spherical beads (usually about 20-150 microns in diameter) of poly(methyl methacrylate) (PMMA) and a small amount of polymerisation initiator such as dibenzoyl peroxide (BPO), usually encapsulated within the PMMA bead, but which can also be added as a separate component. The liquid component is usually a monomer, typically methyl methacrylate (MMA), which may also contain a polymerisation activator such as N, N-dimethyl-p-toluidine (a tertiary amine) (DMPT) and an inhibitor such as hydroquinone (HQ) to prevent the monomer from spontaneously polymerising.
When the solid and liquid components are mixed together, the polymer particles are wetted with monomer, solvated and begin to dissolve. The solvated polymer particles release dibenzoyl peroxide initiator into the monomer which interacts with activator, if present, to produce radicals that react with the monomer and initiate room temperature addition polymerisation of the monomer. The mixture starts out at a relatively low viscosity and progresses to a stiffer and stiffer system that eventually hardens completely.
Compositions hardenable at room temperature (so-called “self-curing” or “cold-curing” systems) have dough times that are typically 4 to 10 minutes and set times that are typically 10 to 25 minutes in duration. The work time effectively defines the time period available for the operator to manipulate the dough in the desired fashion, for example pressing into a denture mould for denture base manufacture, or pressing into a bone cavity during hip repair or replacement or injecting into a vertebral cavity during spinal surgery or forcing into a gap or cavity during industrial cementing operations. The dough time is determined by the rate at which the combination of solid and liquid components rise in viscosity immediately after mixing and is controlled by a number of factors, such as polymer bead particle size and shape, polymer molecular weight, and polymer composition.
Radiopacifying fillers such as barium sulphate or zirconium dioxide are a necessary ingredient to add to bone cement compositions. They function as X-ray contrast agents to show the location of bone cements when implanted in the body. Radiopacifying fillers may traditionally be added to a solid-liquid bone cement composition in either the solid component or liquid component, or both. However, the introduction of filler particles has a tendency to reduce the mechanical properties of the hardened composition such as flexural, tensile and fatigue properties.
U.S. Pat. No. 4,500,658 discloses that a problem with certain types of metal filler such as lead foil, silver alloy, gold and 1% set amalgam are that they cause stress concentrations at the interface between the insert and the resin which weakens and fractures the material.
On the other hand, heavy metal compounds externally attached to the beads are inconvenient. In addition, it is pointed out that high levels of barium sulphate are necessary in the resin to render it radiopaque but that at these levels there is a negative impact on the strength of the material. The document describes that the filler can be encapsulated and uniformly dispersed in the bead using suspension polymerisation. The beads are said to be useful in biomedical applications to colour biomedical materials and devices. An example is proposed of compounding the bead particles into a dry powder for a solid-liquid denture composition. The beads may be ground up prior to use in a composition. The mixtures with monomer disclosed imply that most of the bead is dissolved in the monomer thereby allowing encapsulated radiopaque pigment to disperse in the monomer. Accordingly, the document merely teaches a manner of opacifier delivery to the matrix of the final polymer.
EP0218471 teaches that barium sulphate radiopacifier incorporated into ethyl methacrylate polymer beads can give improved mechanical properties with n-butyl methacrylate monomer in a solid-liquid system. There is no mention of the possibility to reduce the concentration of radiopacifying filler particles through encapsulating within acrylic polymer bead particles. Furthermore, upon mixing the bead polymer is said to be almost fully dissolved in monomer so that there is no intention to maintain encapsulation of radiopacifying filler in the final product. It is one object of the present invention to provide bone cements and dental compositions with improved mechanical properties.